Radio frequency distribution system



Dec. 15, 1936. c. c. sg-UMARD 2,054,227

RADIO FREQUENCY DISTRIBUTION SYSTEM X y Original Filed Feb. 25. 15251 2 Sheets-Sheet 1 mumnu xi INVENTOR CHARLES c. snuum ATTORNEY Dec. 15, 1936. c. c. SHUMARD RADIO FREQUENCY DISTRIBUZIION SYSTEM Uriginal Filed Feb'. 25. 1931 2 Sheets-Sheet 2 INVENTOR CHARLES C. S UMARD f90-UW ATTORNEY Patented Dec. 15, 1936 RADIO FREQUENCY DISTRIBUTION SYSTEM Charles C. Shumard, Moorestown, N. J., assgnor to Radio Corporation of America, a. corporation of Delaware Application February 25, 1931, Serial No. 518,093 Renewed May 27, 1936 3 Claims.

My present invention relates to electrical distribution systems, and more particularly to an improved electrical distribution system for distributing modulated radio frequency energy.

There has been disclosed by J. Weinberger in application Serial Number 350,869 filed March 29th, 1919, Patent No. 2,003,962, granted June 4, 1935, a radio frequency distribution system especially adapted for apartment houses, hotels and the like, wherein modulated radio frequency energy collected on a single antenna may be distributed to a plurality of translating devices, or receivers, located at acoustically isolated points. As pointed out in this aforementioned application it has been found by apartment house tenants particularly, that good reception cannot be procured with the usual receiver on an indoor loop antenna because shielding effect of the building was too great, whereupon each tenant customarily erected his individual outdoor antenna. The resulting accumulation of individual antennae upon the premises of a singie building were so objectionable that one of the main objects of the Weinberger invention was to provide a radio frequency distribution system wherein a single collector of modulated radio frequency energy could be utilized for the energization of a plurality of separated radio receivers. However, it was found that the mere coupling of a plurality of radio receivers to a single antenna was not sufficient; it being discovered that annoying disturbances, and even considerable loss of reception emciency, was had. Accordingly, the aforementioned application pointed out means whereby a plurality of inde? pendent radio receivers could be coupled to a single radio frequency energy collector without frequency discrimination, and without interac- 40 tion between individual radio receivers.

Specifically, this was accomplished by providing a distribution system connected through an amplifier coupling unit to the single antenna, and coupling to the distribution system the individual radio receivers. The distribution system comprised a plurality of two-wire aperiodic transmission lines, each line being preferably made aperiodic by loading it with inductances; that is, this method of maling a line aperiodic 50 was employed to transfer all frequencies within the broadcast range with substantially equal efficiency.

Now, I have devised an improved arrangement for collecting modulated radio frequency energy, 55 preferably in the broadcast range, and aperiodically distributing such collected energy to a plurality of independent, or acoustically isolated, receiver devices, the distributing system employed in the present invention essentially comprising a single conductor cable.

Accordingly, it is one of the main objects of my present invention to provide an improved radio frequency distribution system which consists of a single antenna feeding, by means of a voltage power amplifier, a shielded single conductor cable as a radio frequency transmission line, the natural impedance of which line is extremely low.

Another important object of the present invention is to provide .en improved radio frequency distribution system comprising a single antenna, a shielded single conductor, lead-covered cable functioning as a radio frequency transmission line, and a voltage power amplier between the antenna and the transmission line consisting of a plurality of stages which are coupled by means of individual tuned air-core radio frequency transformers.

Another object of the present invention is to provide a radio frequency transmission line for a radio frequency distribution system which line comprises a single insulated conductor disposed within a metallic sheath, there being a metallic ground strip disposed between the sheath and the insulated conductor.

Still another object of my present invention is to provide in an improved radio frequency distribution system comprising a single antenna, a voltage power amplifier and a transmission line, a plurality of outlet devices, each outlet being adapted for connection to the input circuit of a radio frequency receiver and comprising a series connection of a resistor and capacitative reactance associated with the high potential side of the said transmission line.

And still other objects of the present invention are to improve generally the simplicity and efficiency of electrical distribution systems, and to particularly provide an improved radio frequency distribution system which is not only durable and reliable in operation but economically manufactured and installed. I The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organizaton and method of operation will best be understood 4by reference to the following description taken in connection with the drawings in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

ln the drawings,

Fig. 1 diagrammatically shows a radio frequency distribution system embodying the present invention,

2 shows a cross sect-ion of the cable employed as a transmission line in the present system.

Fig. 3 is an isometric view, partly broken away, and showing the construction of the cable in Fig. 2,

Fig. 4 shows a modified form of transmission line.

Referring to the accompanying drawings in which like characters of reference indicate the same parts in the different figures there is diagrammatically shown in Fig. l acentral antenna, or energy collecting device, I. Although not specifically shown in Fig. 1, it should be understood that the central antenna is usually mounted on the roof of the structure containing the plurality of receiver devices. The energy collector I supplies the collected radio frequency energy to a. plurality of translating devices, or receivers (not shown), which receivers are located in different apartments or, in general. in differentrooms of the same apartment. Each receiver is coupled to one of a plurality of twowire aperiodic transmission lines generally denoted by the numeral 20. Each line 29 is coupled to the common collector I through a voltage power amplifier unit 2l.

Each voltage power amplifier 2I comprises a plurality of elements to now be described in detail, special reference being made to the unit disposed between the antenna I and the most adjacent portion 2-'3 of the transmission line, it being clearly understood that every other unit throughout the entire system is similarly constructed. As shown, the antenna I connected betwn the control electrode and cathode of the electron discharge tube I3 through a resistor 6 disposed in the antenna, the resistor having preferably a value of 30,000 ohms. The anode of the tube I3 is coupled to the input circuit of the tube I4 through an air-core radio frequency transformer II. It will be observed that the second stage of the power amplifier comprises the two tubes I4 and I5 connected in parallel, the output circuit of the tube I5 being coupled. by means of an air-core radio frequency transformer I6, to the input of the transmission line 20', 20.

Each voltage power amplifier, furthermore, includes a socket power unit comprising a power source transformer I having its primary coil I' adapted for connection to any commercial power line, while its secondary includes the usual three portions, 2. 3 and 4. The anodes of the double wave rectifier tube I3 are connected to the secondary portion 3', whiie the cathode of the rectier tube is connected to the portion 2'. The cathodes of the amplifier tubes I3, I4 and i5 are connected for heating through the cone ductors 3' to the secondary portion 4. Voltage for the anodes of the tubes is secured from the conductor I4'. while the control electrodes of each of the amplifier tubes is connected to the conductor I5'. the latter being connected to the midpoint of the secondary portion 3'.

A line lter made up of the shunt capacities 2 and 3, and the series choke coil 4 is disposed between the rectifier tube and the amplier tubes, and a resistor 5 having a value of about 70,000 ohms is connected in shunt with the capacity 3, and in series with the choke coil The cathodes of all the amplifier tubes have connected across them the resistor l, having a value of about 60 ohms, which is tapped at its electrical midpoint, the midpoint being connected by means of a resistor 8, having a. value of about 540 ohms, to the conductor I5'. A series connection of two capacities 9 and 9', each having a value of about 0.2 microfarad, is disposed between the conductor II!i and one side of the resistor 8, a point between the two capacities being connected to the other side of the resistor 8.

It will be noted that the voltage power arnplifier comprises two stages of amplification, and that the air-core radio frequency transformer II hw a radio frequency by-pass capacity II. The secondary coil I6' of the output radio frequency transformer of the unit 2l feeds into the portion 20' of the transmission line. Shielding of the complete amplifier 2l is not necessary for stabilization, the actual necessity being to shield only the radio frequency transformer. However. it is preferred to shield the radio frequency portions of the circuits to the antenna lead-in in order to minimize stray pick-up. An electrostatic shield is therefore made a part of the power transformer, and a line lter is installed in troublesome locations.

Also, in troublesome locations, it is advisable to keep the antenna lead-in short by placing the amplifier unit or units, as close to the antenna as advisable. As a matter oi fact, a shielded transmission line of the type to be hereinafter described could be used with effectiveness as the lead-in, where itis not advisable to make thev A lead-in short.

An electrostatic shield 30 is, thereforc shown surrounding the unit ZI, and connected to ground and by a conductor 3I to the core I0 of the power transformer. It will also be observed that the shield is grounded at 32, through a connection 33, between the ground and conductor i5'. All amplifier radio frequency circuits are isolated by appropriate by-pass condensers. Coupling between the stages in the amplifier is obtained by means of individual untuned air-core radio frequency transformers II and I6, both primary and secondary of each transformer being untuned.

The output of the voltage power amplifier feeds into a standard. shielded, single, conductor, lead-covered cable 20 functioning as a radio frequency transmission line, the natural impedance of which line is approximately 50 ohms. The line may be used electrically the same either in, or out. of conduit. The construction of the radio frequency transmission line will now be described in detail, special reference being made to Figs. 2 and 3.

The transmission line consists of a single conductor 40 comprising preferably forty-one strands of #29 copper wire. This conductor is covered with several layers I of insulation material, the latter preferably being five layers of unwaxed cotton braiding. The insulated conductor is disposed within a metallic sheath 42, preferably leadI while a metallic ground strip 43 preferably copper, Is disposed between the insulated conductor and the metallic sheath. The ground strip lies in electrical and mechanical contact with the lead sheath, and is given a slight curvature to conform with the conductor and braiding 4I, the strip running the length of the cable, and

preferably having a width of 0.15 inch and a thickness of 0.2 inch. It has been found that the ground strip and sheath greatly minimize cable attenuation. Of course. the showing of the cable in Figs. 2 and 3 is greatly magnified for the purpose of illustration.

At desired points of each transmission line 20 there are arranged receiver outlet taps. Thus, the numeral 50 designates an outlet tap, or box. comprising a metallic housing which is grounded by means of a connection 5| to the ground side of the transmission cable. which is the ground strip 43. The conductor 60, or the antenna side of the transmission cable. is connected to the antenna terminal of a radio receiver through a series connection of a capacity I1, preferably having a value of 300 micro-microfarads, and a resistor 26, preferably having a value of 500 ohms. The reference letters A and G designate the terminals to which the antenna and ground terminals of a radio receiver are to be connected when it is desired to connect such a receiver to the transmission line 20.

The 300 micro-microfarad condenser is used in order that all sets may be operated mechanically the same, and electrically practically the same, from the transmission line as from an average antenna, and at the same time not affect appreciably either line conditions or other receivers operated on the line 2U when it would be connected, disconnected, or operated. This condenser. also, serves to isolate the receiver input circuits so far as direct current is concerned. The 500 ohm resistor is utilized so that in the case of receivers having series tuned input circuits short-circuit of the transmission line, depending upon the degree of tuning and the resistance of the circuit, may not occur. 'I'he lower receiver is shown as having such a series tuned input circuit 21.

In Fig. l there have been shown three outlet taps 5D connected to one of the transmission lines 20. and it will be seen that all the outlet taps are of the same construction. with the exception of the last outlet. This latter outlet. whenever occurring at the end of a given transmission line usually includes a terminating resistor i8 connected between the ground strip and the conductor. the resistor preferably having a value of about 50 ohms. Of course, each transmission line may be terminated' by a resistor I8 without employing an outlet tap connection for a receiver.

A second transmission line is shown. and it will be seen that the outlet taps 50 in this line, are preceded by a voltage power amplifier unit 2|. The specic construction of the unit 2| and the two outlet taps 50 in this second transmission line have not been shown for the reason that they are exact duplicates of the power amplier unit and outlet taps heretofore described.

In Fig. 4 there is shown a transmission cable which is yslightly modified from that shown in Figs. 2 and 3. This type of transmission line includes loading coils 40'. the terminals of each loading coil being connected to the conductor lll. the diameter of the lead sheath 42 being widened to receive the space loading coils. as at 42', and each loading coil being spaced by insulating material d3' from the lead sheath.

The maximum recorded length of cable to be used with any one amplifier 2| is 500 feet. and radio sets may be connected at any desired point in the line by means of the outlet tap described. If it is desired to line more than one riser to more than one antenna, as many as three ampliner units may be connected in parallel their respective grounds being connected in common. If it is desired to run a line longer than 500 feet. another amplifier may be connected at any point in the transmission line and an additional 500 feet of transmission line employed with its associated outlet units and line terminating resistor unit.

It should be particularly noted that the transmission cable provided herein is especially designed for use where uniform capacitance and inductance between the conductor and groundare required, the specific form in the present case being shown as a exible, insulating cable having a copper ground strip and covered with an outside sheet of lead. This type of cable readily solves the lproblem of economical radio frequency distribution. The lead sheath of the cable and copper strip which is in contact with the lead sheath should be well grounded to the cold water supply line at a point as close to the earth as possible. For example. if a cable terminates in the ground floor or basement apartment, a good ground connection employing #10 copper wire should be soldered to the sheath and copper ribbon, and run from this point on the cable to the cold water supply line in the basement.

While the transmission cable has been shown as including a lead sheath. it is to be clearly understood that the use of such a sheath is not absolutely essential. it being possible. for example, to employ a cloth metallic braid. or even the ground strip .secured to the insulation material 4| in any desired way. Furthermore. while I have indicated and described several systems for carrying my invention into eect. it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described. but that electrically equivalent modications in the circuit arrangements. as well as in the apparatus employed, may be made without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. In a system for the distribution of radio frequency energy. an energy collecting means. an

apex-iodio amplifier tube coupled thereto'. a two wire aperiodic transmission line coupled to the output circuit of said tube. means for grounding one side of said line. a plurality of outlet taps situated at spaced apart points along said transmission line, a plurality of radio receivers connected to said outlet taps at least one of said receivers having a series tuned circuit connected across two of said outlet' taps and means to prevent the tuning of said circuit from substantiallv lowering the voltage of the high potential side of said line. said means comprising the series connection of a resistance of the order of f conductors and arranged in metallic contact with said ground conductor. short conductive connections between certain of said outlet taps and said ground conductor. connections between the others of said outlet taps and said insulated conductor each of said connections including the series connection of a condenser and a resistor having a value of substantially 500 ohms. and conductive connections between said ground conductor and ground at a pluralityf said outlet taps.

3. In combination in a system for the distribution of a broad band of radio frequency currents, an aperiodic amplifying device having L input and output circuits. a two wire apeidic transmission line connected to the output circuit of said amplier, a plurality oi metallic outlet boxes at spaced apart points on said line, means for conductively connecting each o! said boxes to one wire of said transmission line. an outiet terminal mounted in each oi said boxes and means including a resistor having a value greater than 200 ohms connecting said outlet terminals to the other wire of said line said connecting means having a high impedance for direct current and a low impedance for currents oi radio frequency.

CHARLES C. SHUMARD. 

